I'm certain it's a one time cost. Though I suspect that given that Closure [optimizes out unused code](http://blog.fogus.me/2011/07/21/compiling-clojure-to-javascr...) the more I use of the goog namespace, the larger that baseline will become.
That being said, 30k is the size of minified jQuery, so folks that are alright with building apps on jQuery won't find 30k to be that terrible.